With the development of the information society, the volume of communication information tends to increase dramatically. To achieve high speed, large capacity and long distance transmission in optical fiber communication is required and indispensable.
As an approach to such high speed and large capacity transmission, it has been carried out the optimization of characteristics of optical fibers used in optical transmission lines. Further, as another approach for long distance transmission, an optical fiber amplifier (OFA) has been developed, which is capable of amplifying an optical signal as it is by adapting a rare earth element-doped fiber, such as an erbium-doped optical fiber (EDF) formed by doping erbium ions into core of an optical fiber, or the like. With the development of these technologies, the high speed, large capacity and long distance transmission in optical fiber communication has made rapidly progress.
On the other hand, in order to enlarge transmission capacity in optical communication, it has been widely developed a communication by wavelength division multiplex transmission (WDM) for transmitting optical signals having different wavelengths through only one optical fiber. By adapting the OFA to the optical signal system using the WDM transmission, it is expected that the transmission capacity in optical communication can be enlarged and the long distance transmission of optic signals can be achieved.
As a representative example of the OFA, there is an optical fiber amplifier (EDFA) using the EDF. Recently, it was attempted that by using such EDFA, the WDM transmission was carried out under a specific transmission bandwidth having, for example, a wavelength ranging from 1520 to 1620 nm as a gain bandwidth of the EDFA.
When the WDM transmission is carried out by applying the EDFA, it is required to decrease the wavelength dependency of the gain of the EDFA as much as possible. As a technique for practically satisfying this need, it has been known to dope specified elements other than erbium ions, for example, aluminum, to the EDF (Refer to S. B. Poole, “Fabrication of Al2O3 co-doped optical fibers by a solution-doping technique” ECOC'88, P433, 1988).
Further, in case of carrying out the WDM transmission by using the EDFA, it is also required that the energy conversion efficiency of the EDF is retained as higher as possible. This requirement can be accomplished by increasing the doping amount of erbium. However, it is generally known that, when rare earth ions aggregate, there occurs a degradation of energy conversion efficiency caused by rare earth ions, a so-called concentration quenching. As one example of the technique to prevent such concentration quenching, it is known to combine and dope a SiO2 glass with rare earth oxides and one or both of phosphorus and/or aluminum more than 10 times than the rare earth oxides in a mole ratio relative to the rare earth oxides.
As known from these conventional techniques, it can be understood that an optical fiber doped with the rare earth element and aluminum has good prospects as it improves the amplification characteristics of the EDFA.
Further, in order to dope a silica glass with the rare earth element and aluminum, the following techniques are known:
(1) a technique for immersing an aggregate of fine silica glass particles obtained by a vapor phase deposition method in an alcohol solution prepared by dissolving aluminum chloride and rare earth compound, then heating and sintering the same; and
(2) a technique for immersing an aggregate of fine silica glass particles containing aluminum oxides obtained by a vapor phase deposition method in a solution of a rare earth compound, then heating and sintering the same.
However, conventional methods for doping the rare earth element and aluminum to the silica glass have the following problems.
According to the above-mentioned method (1), it is described that the doping of aluminum can be stably conducted. However, when it intends to dope a large quantity of aluminum by this technique, doping amount of the aluminum is limited since the aluminum is not evenly distributed.
Further, according to the above-mentioned method (2), it may be possible to dope plenty amount of aluminum. However, after the silica glass is immersed with the alcohol solution containing dissolved erbium chloride then heated and sintered, the aluminum is not efficiently provided around erbium. Thus, in the case that a large quantity of erbium is doped so that the concentration of erbium in glass can be over 500 mass ppm, it is impossible to completely control the occurrence of concentration quenching.
Additionally, the aggregate of fine silica glass particles doped with aluminum and erbium by the above-mentioned method (1) or (2) is prepared by the following process. That is, the aggregate of fine silica glass particles is heated at 70 to 100° C. under an inert gas atmosphere, dried by evaporating the alcohol component acting as a solvent in the solution contained in the aggregate, and then vitrified into a transparent glass in a high temperature.
Thus, however, by heating at 70 to 100° C. under an inert gas atmosphere, the aggregate is dried by removing alcohol component and then made into a vitrified glass. However, the crystal water cannot be removed sufficiently from the aggregate by the heating at 70 to 100° C. under an inert gas atmosphere. Accordingly, such crystal water or residual solvent component contained in the solution, moisture in the air and the like are rapidly evaporated by rapidly heating the aggregate upon the vitrification process. By such rapid evaporation, the aggregate of fine silica glass particles is damaged to cause a crack. Further, the rapid heating upon vitrification process expands a small crack occurred upon drying the aggregate, and enlarges the crack around the surface after vitrification process.
Further, in consideration of residual percentage, which is a ratio of the amount of co-dopant such as aluminum chloride introduced into the solution to the amount of co-dopant such as aluminum contained in the final glass, there is the following problem. That is, by rapidly exposing the dried aggregate to a high temperature of greater than 1000° C., most of the co-doping substances are volatilized thus the final residual percentage is substantially decreased, thereby causing the deterioration of yield.
Further, in case of utilizing a variety of dopants of various kinds, ability of glass formation of the silica glass is sharply decreased thereby occasionally inducing a change in glass structure in a high temperature treatment or further process. Especially, when aluminum is doped to an EDF in a great amount, crystals such as cristobalite or mullite generate at such a low temperature that cannot be expected in a typical optical fiber preform.
The use of aluminum vapor doping method enables the doping of a large quantity of aluminum. However, as the doping amount of aluminum becomes larger, it cannot avoid the crystallization to be occurred in the same process as for the typical optical fiber preform, and due to the resulting bubbles or so on caused by crystals, it is difficult to make the optical fiber preform.
Accordingly, it is an object of the present invention to provide a stably doping means for doping a glass with a large quantity of a rare earth element and dopants for improving the functionality of the rare earth element.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.